Copper is widely used as a wiring material because of its high electrical conductivity next to silver among all metals and inexpensiveness. For example, a technique of forming a circuit by forming a copper layer on a substrate and removing an unnecessary copper portion by means of etching has been classically used.
However, according to this method, because not only of a large number of treatment steps but also of a necessity of a treatment of an etching waste liquid, there was involved such a problem of a lot of costs and large in the load on the environment.
Meanwhile, put into practical use is a technique of forming a circuit by kneading copper particles and a resin binder with a solvent or the like to process into a paste and printing this, followed by heating and burning, whereby the resin binder is cured to keep contact of the copper particles with each other. However, according to this method, since a relatively large amount of the resin binder remains within a conductor even after forming a circuit, it was difficult to achieve sufficient electrical conductivity.
In addition, there is a known phenomenon in which by making metallic particles small in a nano-level size, the metallic particles are joined with each other by heating at relatively low temperature as if they melt together. Thus, developed is a technique in which copper particles are joined with each other by printing and heating and burning by utilizing this phenomenon, to form a circuit, thereby reducing a resin binder remaining within a conductor after the circuit formation. However, according to this method, since the copper particles must be processed into a nano-level size, there was involved such a problem that the manufacturing costs become high.
Meanwhile, a method for forming a circuit by printing a wiring pattern by using a copper composition capable of depositing copper by heat decomposition, followed by heating to deposit copper is proposed. According to this method, since it is not necessary to process the copper particles into a nano-level size, the manufacturing costs can be reduced. In addition, since a resin binder remaining within a conductor after forming a circuit can be reduced, good electrical conductivity can be achieved.
However, in a copper film formed by using such a technique, since only a small amount of or substantially no resin binder which contributes to adhesion to a base material is contained, the adhesion to the base material is ensured exclusively by a direct interaction between copper and the base material surface. Since copper is originally hydrophilic and the adhesion on the hydrophobic surface is not ensured, it is desirable that the base material surface is hydrophilic. For that reason, a composition that is aimed at being used in this application is required to have excellent affinity with the hydrophilic surface.
In addition, in order to reduce a heat load to the base material and to reduce the energy consumption, it is required that copper can be deposited at a lower temperature, specifically, copper can be deposited at a temperature of 130° C. or lower at which it becomes possible to apply to a polyethylene terephthalate film.
Patent Document 1 discloses a method of depositing metallic copper by heating a composition containing copper, two formate ions coordinated to the copper, and two C9-C20 alkyl imidazoles coordinated to copper through nitrogen. However, the adhesion to a base material and the pattern formation are not mentioned, and the technique for circuit formation is not disclosed. In addition, this composition is aimed at being used in a supercritical fluid such as supercritical carbon dioxide, etc., and its use at atmospheric pressure is not shown and it is not suitable for the formation of a copper film on a hydrophilic surface because of its high hydrophobicity.
Patent Document 2 discloses a method of depositing a copper film by heating a mixed product composed of a copper formate and an alkoxyalkylamine. However, the adhesion to a base material is not disclosed and the formation of a copper film at 130° C. or lower is not described.
Patent Document 3 discloses a method for manufacturing a copper film by heating a copper compound composed of a copper formate and ammonia. However, the formation of a copper film at 130° C. or lower is not described.
Patent Document 4 discloses a method for manufacturing a copper film by heating a copper precursor composition obtained by blending a copper formate and a propanediol compound. However, the formation of a copper film at 130° C. or lower is not described.
In addition, Non-Patent Document 1 describes a purification of a dimeric copper (II) complex with 1-methylimidazole by adding a copper formate to an ethanol solution containing excessive 1-methylimidazole. However, the contents thereof are concerned with a consideration regarding chemical structures, but a description regarding the formation of a copper film is not seen.